NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have already put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches focuses on removing NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, selective catalytic reduction (SCR), and lean NOx traps (LNTs).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Currently, peak NOx conversion efficiencies for lean-burn catalysts are unacceptably low. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more.
Ammonia-SCR refers to selective catalytic reduction of NOx by ammonia. Often, this is referred to simply as SCR. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
LNTs are NOx adsorbers combined with catalysts for NOx reduction. The adsorbant is typically an alkaline earth oxide adsorbant, such as BaCO3 and the catalyst is typically a precious metal, such as Pt or Ru. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. Accumulated NOx is removed by creating a rich environment within the LNT through the introduction of a reductant. In a rich environment, the catalyst activates reactions by which adsorbed NOx is reduced and desorbed, generally as N2.
A LNT must periodically be regenerated to remove accumulated NOx. This type of regeneration may be referred to as denitration in order to distinguish desulfation, described below. The conditions for denitration can be created in several ways. One approach uses the engine to create a rich fuel-air mixture. For example, a spark ignition can be run rich. A diesel engine can inject extra diesel fuel into the exhaust of one or more cylinders prior to expelling the exhaust. Reductant may also be injected into the exhaust downstream of the engine. Where the engine is run lean, a portion of the reductant is generally expended to consume excess oxygen in the exhaust.
Reductant can consume excess oxygen by either combustion or reforming reactions. Typically, the reactions take place upstream of the LNT over an oxidation catalyst or in a reformer. The reductant can also be oxidized directly in the LNT, but this tends to result in faster thermal aging.
U.S. Pat. Pub. No. 2003/0101713 describes an exhaust system with a fuel reformer placed inline with the exhaust flow upstream of a LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen and converts diesel fuel reductant into more reactive reformate.
Many publications propose reducing the fuel penalty by providing two or more branches in an exhaust aftertreatment system. During regeneration of a LNT in one branch, all or part of the exhaust flow can be diverted to the other branch. The methods described in these publications require the use of at least one exhaust valve that for a heavy duty truck must generally fit an exhaust pipe with an inner diameter of at least about 10 cm. U.S. Pat. No. 6,820,417 describes a four-way valve for this purpose. U.S. Patent Pub. No. 2004/0139730 describes a valve that divides reductant and exhaust between two LNTs. In a first position, the valve directs reductant to one LNT and exhaust to the other. In a second valve position the flows are switched. The durability and reliability of these valves is not known, although experience with smaller EGR valves suggests durability and reliability will present challenges for these valves.
U.S. Pat. No. 6,735,940 proposes a dual leg system in which the flow is balanced between the two branches except during regeneration. Each branch contains a LNT and an igniter. During regeneration, using a three-way valve, 80% of the flow is directed to one branch while 20% of the flow is directed to the branch being regenerated. Fuel is injected into the branch being regenerated. Part of the fuel is burned by the igniter to eliminate excess oxygen. Reducing the flow during regeneration is said to result in a substantial reduction in fuel penalty. The flows are united downstream and treated by an oxidation catalyst, which is said to virtually eliminate hydrocarbon emissions. U.S. patent Pub. No. 2004/0037755 describes an alternative embodiment wherein one branch contains a LNT and the other branch is a simple bypass.
PCT Pub. No. WO 2004/020807 proposes to avoid the use of valves in a dual leg system by providing two exhaust manifolds, with half the cylinders exhausting into one manifold and the other half into the other. Each manifold channels exhaust into its own exhaust branch and each branch contains a LNT. The exhaust branches unite downstream of the LNTs into a trailing conduit that contains an oxidation catalyst. The LNTs are regenerated alternately and the system is controlled to assure the exhaust in the trailing conduit is always oxygen rich, whereby the oxidation catalyst is continuously active to oxidize hydrocarbons, carbon monoxide, and hydrogen sulfide even without a large oxygen storage capacity.
Hydrogen sulfide can be released by a LNT during desulfation. Desulfation is the process of removing SOx which, like NOx, accumulates in the LNTs. SOx is the combustion product of sulfur present in ordinarily diesel fuel. Even with reduced sulfur fuels, the amount of SOx produced by diesel combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere.
A LNT can produce ammonia during denitration. This ammonia can be oxidized, like H2S, however, ammonia can be advantageously captured by a downstream SCR catalyst for subsequent use in reducing NOx, thereby improving NOx conversion efficiency with no increase in fuel penalty or precious metal usage. U.S. Pat. No. 6,732,507 describes a system with an ammonia-SCR catalyst configured downstream of a LNT for this purpose. U.S. Pat. Pub. No. 2004/0076565 describes such systems wherein both components are enclosed by a single shell and/or co-disbursed. PCT Pub. No. WO 2004/090296 describes such a system wherein there is an inline reformer upstream of the NOx adsorber-catalyst and the SCR catalyst.
U.S. Pat. No. 5,727,385 describes a system in which a hydrocarbon-SCR (HC-SCR) catalyst is configured upstream of a LNT. The two components together are said to provide higher NOx conversion than either of the components individually.
U.S. Pat. No. 6,677,264 describes a combined LNT/HC-SCR catalyst. The catalyst comprises two layers on a support. The first layer is a NOx adsorber-catalyst and the second layer is an HC-SCR catalyst having a HC-storing function provided by a zeolite. The HC-storage function is intended to concentrate hydrocarbon reductants in the vicinity of the catalyst and thereby increase activity.
U.S. Pat. No. 6,202,407 describes an HC-SCR catalyst that has a hydrocarbon-storing function. In one embodiment, a diesel fuel reductant supply is pulsed and the catalyst continues to show activity for extended periods between the pulses.
U.S. Pat. No. 5,603,216 describes a branched exhaust system in which one branch contains a hydrocarbon-adsorbing zeolite and the other branch is empty. The two branches unite into a catalytic converter. Secondary air injectors are provided, which are intended to allow selective diversion of the exhaust into the branch containing the zeolite during warm-up. After the zeolite and the catalytic convert have warmed, the secondary air jets are turned off and the exhaust flows through the empty branch.
In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust aftertreatment system that is durable, has a manageable operating cost (including fuel penalty), and can practically be used to reduce NOx emissions across the spectrum of diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.